The term “attrition mill” is herein used to include mills used for fine grinding for example, stirred mills in any configuration such as bead mills, peg mills; wet mills such as colloid mills, fluid energy mills, ultrasonic mills, petite pulverisers, and the like grinders. In general, such mills comprise a grinding chamber and an axial impeller having a series of mainly radially directed grinding elements such as arms or discs, the impeller being rotated by a motor via a suitable drive train. The grinding elements are approximately equally spaced along the impeller by a distance chosen to permit adequate circulation between the opposed faces of adjacent grinding elements and having regard to overall design and capacity of the mill, impeller speed and diameter, grinding element design, mill throughput and other factors.
Such mills are usually provided with grinding media and the source material to be ground is fed to the mill as a slurry. Although the invention is herein described with particular reference to the use of various forms of grinding media added to the mill, it will be understood that the invention may be applied to mills when used for autogenous or semi-autogenous grinding. In the case for example of a stirred mill used for grinding pyrite, arseno-pyrite, or the like, the grinding medium may be spheres, cylinders, polygonal or irregularly shaped grinding elements or may be steel, zircon, alumina, ceramics, silica-sand, slag, or the like. In the case of a bead mill used to grind a sulphide ore (for example galena, pyrite) distributed in a host gangue (for example, shale and/or silica) the gangue may itself be sieved to a suitable size range, for example 1-10 millimeters or 1-4 millimeters, and may be used as a grinding medium. The media size range is dependant on how fine the grinding is required to be. From about 40% to about 95% of the volume capacity of the mill may be occupied by grinding media.
It should be recognized that in the grinding process, grinding media undergoes size reduction as does source material to be ground. Grinding media which is itself ground to a size no longer useful to grind source material is referred to as “spent” grinding media. Grinding media still of sufficient size to grind source material is referred to as “useful” grinding media.
A source material to be ground, for example a primary ore, mineral, concentrate, calcine, reclaimed tailing, or the like, after preliminary size reduction by conventional means (for example to 20-200 microns), is slurried in water and then admitted to the attrition mill through an inlet in the grinding chamber. In the mill, the impeller causes the particles of grinding media to impact with source material, and particles of source material to impact with each other, fracturing the source material to yield fines (for example 0.5-90 microns). It is desirable to separate the coarse material from the fines at the mill outlet so as to retain useful grinding media and unground source material in the mill while permitting the fines and spent grinding media to exit the mill.
In some attrition mills, outlet separation is achieved by means of a perforated or slotted screen at, or adjacent to, the mill exit and having apertures dimensioned to allow passage of spent grinding media and product but not permitting passage of useful grinding media. For example, if it is desired to retain particles of greater than 1 mm in the mill, the outlet screen aperture width would be a maximum of 1 mm so that only particles smaller than 1 mm would exit the mill through the screen. The outlet may in addition comprise a scraper or a separator rotor to reduce screen clogging. The axial spacing between the facing surfaces of the separator rotor and the last downstream grinding element is approximately equal to the spacing between the facing surfaces of all the other pairs of grinding elements.
The design and operation of attrition mills and media selection is highly empirical. Although various mathematical computer-based models have been proposed, none have yielded satisfactory predictions of mill performance.
In attempting to finely grind a sulphide ore using various grinding media in a high throughput bead mill e.g. having a mill throughput of greater than 10 TPH, it was found that the outlet screen rapidly clogged reducing the throughput to an intolerably low level. Moreover, the rate of wear of the separator rotor and outlet screen rendered operation uneconomic.
U.S. Pat. No. 5,797,550, the entire contents of which are incorporated herein by cross reference, describes an attrition mill having improved means for classification and/or separation of coarse particles from fine particles in a slurry. The attrition mill described in this patent comprises a grinding chamber, an axial impeller, a chamber inlet for admitting coarse particles, and a separator comprising a chamber outlet through which fine particles exit from the chamber. The mill is characterised in that a classification between coarse and fine particles is performed in the mill upstream of the separator. By conducting classification between fine and coarse particles upstream from the mill outlet, the maximum size of particles exiting from the mill is substantially independent of the minimum orifice dimensions of the chamber outlet.
Classification may take place in this mill by providing a classifier element defining a first surface in rotation about an axis, a second surface spaced from and facing the first surface so as to define a passage there between, a classifier inlet for admitting slurry to the passage, a first classifier and outlet spaced from the classifier inlet whereby the slurry exits from the passage, a second classifier outlet spaced radially outwardly of the classifier inlet, and means for causing the slurry to flow from the classifier inlet to the first classifier outlet at a predetermined volumetric flow rate. The first surface is spaced sufficiently closely to the second surface and is rotated at sufficient speed so that a majority of the particles in the passage having a mass of less than a predetermined mass remained entrained with slurry flowing into the first classifier outlet and a majority of the particles exceeding a predetermined mass are disentrained and move outwardly from the passage at the second classifier outlet.
The passage may be defined between two members which may be rotated (or counter rotated) independently of the axial impeller and/or of each other.
The attrition mill of this patent may also include a separator stage comprising a separator rotor mounted to the impellor and spaced axially from an endplate to define a radially extending separation passage therebetween, said first classifier outlet admitting slurry to the separation passage at a radially inner region of the separator element, baffle means at or near the separation passage periphery to permit passage of coarse particles travelling outwardly to beyond the separation passage periphery, and a slurry outlet spaced axially from the radially extending separation passage to permit passage of the fine particles out of the mill. The baffle means may be in the form of axial fingers positioned around the periphery of the separator rotor and extending towards the chamber outlet.
The attrition mill described in U.S. Pat. No. 5,797,550 is commercially available from the present applicant and is sold under the trademark ISAMILL™.
It is known that attrition mills, such as the prior art attrition mills described above, include a plurality of grinding discs mounted to a rotating shaft. These grinding discs typically include a series of openings, such as a plurality of equiangularly spaced openings. During use of prior art attrition mills, the slurry circulates through the apertures in the grinding discs and particles also went between facing surfaces of the grinding discs and flung against other particles, against the shaft between the grinding discs, against the disc surfaces and against the mill walls. The slurry circulates a radial direction between the discs and adjacent to the shaft.
The attrition mill is described in U.S. Pat. No. 5,797,550 has proven to be technically and commercially successful.